Membranes are widely used as microsensor elements, vacuum windows, and substrates for lithographic masks. In particular, masks formed on membranes are of interest for fabricating integrated electronic devices by x-ray lithography. In such a process, an electronic substrate is coated with a resist material that is sensitive to x radiation, and the material is exposed in a pattern defined by a mask for incident x rays. The mask generally includes a desired pattern of x-ray attenuating material and a substrate for supporting the attenuating material. However, a body of substantial thickness underlying the patterned region may itself unacceptably attenuate the incident x radiation in areas that should be transmissive. Generally, this problem is avoided by depositing the attenuating material on a thin membrane that is supported at its periphery by a thicker, typically ring-shaped, support region. The membrane is typically 0.5-5 .mu.m thick. Desirably, the membrane is relatively robust; e.g., capable of withstanding, over a 2-.mu.m thickness and 3-cm diameter, a pressure differential across the membrane of at least 20 kPa.
The search for a robust membrane has been fraught with difficulty. For example, one widely investigated x-ray mask is made by forming a heavily boron-doped surface layer on a silicon wafer. A membrane is then formed by exposing the wafer to a selective etchant that removes relatively undoped silicon much more rapidly than the boron-doped silicon. A support ring is defined by applying an etch-resistant material to a peripheral, annular region on the face of the wafer opposite to the boron-doped face. After etching, the thin, boron-doped layer remains suspended within the annular support ting. The etchant must exhibit high selectivity to avoid substantial thinning of the boron-doped layer. The boron-doped silicon is typically single-crystalline on the silicon substrate. As such, it is susceptible to fracture along its crystallographic planes, and therefore does not generally attain the desired degree of robustness.
A more robust membrane is described in U.S. Pat. No. 5,051,326, issued to G. K. Celler, et al. on Sep. 24, 1991. The membrane described therein is made from polysilicon, and is supported by a silicon-oxide-containing body. To make this membrane, a thin, polysilicon region is formed on the surface of a substrate that comprises a silicon-oxide-containing composition such as silica or a silicate. The substrate material is then removed (except for the support ring) by an isotropic etchant that leaves behind the polysilicon membrane. This membrane has a tensile stress of 0.28 GPa and, because of its polycrystalline nature, is less susceptible to fracture than single-crystalline silicon membranes. However, there is a relatively high contact angle, typically greater than 20.degree., between the edge of the membrane and the support ring.
If a membrane of excellent quality is rigidly supported, it will tend to fail at the periphery, where it is attached to the support ring. When the membrane flexes in response, e.g., to a pressure differential, stress will concentrate in this region. The smaller the bend radius is at the edge of the membrane, the greater will be the stress concentration and the tendency to fail. Flexure of membranes occurs during processing as well as routine handling. The consequent tendency of the membrane to fail decreases manufacturing yield and reduces reliability. Improvements such as that described in the above-cited patent have led to increased material strength of the membranes, but have hitherto failed to address the mechanical problem of stress concentration at the membrane edge.